1. Field of the Invention
The present invention relates to a power supply system for a motor vehicle that is mounted on the vehicle and used for charging up the battery and capacitor thereof.
2. Description of the Related Art
An in-vehicle AC generator conventionally used as a power supply system for a motor vehicle includes a system in which output from the generator is stepped down by a DC-DC converter so as to charge up the battery mounted on the vehicle (refer to, for example, Japanese Laid-open Patent Publication No. 2002-95177, hereinafter referred to as Patent Document 1). This system is configured in such a way that the field coil is supplied with power obtained from a low voltage output terminal rather than a high voltage output terminal.
Moreover, there is a system as another conventional example in which, using an electric double-layer capacitor, kinetic energy of the vehicle is converted into electric energy so as to regenerate power (refer to, for example, Japanese Patent Publication No. 3606740, hereinafter referred to as Patent Document 2).
A motor vehicle does not need fuel for rotating the engine when decelerating. At this time, rotation force from the wheels is transmitted through its power transmission path to the engine as rotation force therefor. Storing as electric energy this rotation energy, after converted by the generator, enables reduction in the amount of fuel consumed for generating electricity in a region of accelerating, cruising and the like in which fuel is injected to generate electricity. We have devised a system that, by connecting the electric double-layer capacitor described in Patent Document 2 to the high voltage output terminal of the conventional in-vehicle AC generator described in Patent Document 1, can store electricity and reduce the amount of fuel consumption. At this time, the conventional in-vehicle AC generator is configured to obtain from the low voltage output terminal power to be supplied to the field coil, although it is provided with the high voltage output terminal from which power capable of being supplied to the field coil can be obtained. Therefore, a sufficient current can not be produced in the field coil, causing a problem in that energy can not be fully regenerated in a deceleration region of the vehicle.
Moreover, since power for producing a field current needs to be obtained steadily from the low voltage output terminal, a current always flows through a connector portion provided at the output terminal of a step-down circuit, causing a problem of generating heat at the connector portion due to its contact resistance, which has in turn deteriorated reliability. The connector has been made larger so as to reduce the contact resistance as a countermeasure against the problem, whereby, however, the system itself has increased in size, pushing up the cost thereof.
On the other hand, in order to solve the above-described problem, configuring the system so as to obtain power for producing the field current from the capacitor connected to the high voltage output terminal has enabled a maximum voltage capable of being applied to the field coil to increase, compared to the configuration in which a voltage applied to the coil is obtained from the battery connected to the low voltage output terminal. However, the capacitor self-discharges more than the battery, and in addition, the capacitor has characteristics in which the voltage of the capacitor rises depending on the increase in the electricity amount stored therein, differing from those of the battery in which the voltage thereof does not vary so much depending on the increase in the stored electricity amount. Therefore, when the capacitor voltage is extremely low in such cases as the capacitor is left unused for a long time so that its self-discharging progresses and the capacitor is replaced with an uncharged capacitor at a time of maintenance, a sufficient field current required for generating electricity can not be produced, so that the capacitor cannot be charged up on its own.
Furthermore, when the capacitor voltage is not extremely low, but lower than that of the battery, a larger field current cannot be produced compared to a case in which power for producing the field coil current is obtained from the battery; therefore, a sufficient amount of energy cannot be regenerated when decelerating, causing such problems as opportunities for regenerating energy without consuming fuel would be lost.
Now, in order to solve such problems as described above in that the capacitor cannot be charged up when its voltage is low, a method has been devised in which the capacitor is charged up from the battery using a bidirectional DC-DC converter that can bidirectionally convert power from the capacitor into that to the battery or vice versa. However, the battery has characteristics in which extreme reduction in the amount of electricity stored therein extremely shortens the lifetime thereof, and charging up the capacitor needs a large amount of power, which has therefore brought about such negative effects as the amount of electricity stored in the battery is greatly reduced, and resultantly the lifetime of the battery is shortened.
On the other hand, different kinds of metals contact with each other at the connector and soldered portions of the field coil. Since the generator is located in the engine room, it follows that the generator is in such wet environments as dew forms due to change in the ambient temperature and water is splashed there from road surfaces. If a current flows for a long time through different metals with the contact portion of the different metals being wet, the metals become corroded due to electric corrosion. Since the field coil of the conventional system is always connected to the battery via an excitation control circuit, even when the current flowing through the field coil is interrupted by the excitation control circuit, electric corrosion has developed due to a small leak current.
Moreover, since the capacitor charged up by the generator is in a state of electricity being stored, there also has been a problem in that hot-line work must be performed when the capacitor is disconnected from the circuit at a time of vehicle maintenance and the like.
Furthermore, we intended to start the engine by operating the generator as a motor-generator. In the motor-generator, both armature and field coils are energized to produce magnetic flux so as to generate rotation force by repulsion force produced by the magnetic flux. However, when the capacitor voltage is low, a current cannot flow through the field coil, causing a problem in that the generator cannot be operated as the motor-generator.